Three groups of experiments are proposed to (1) characterize the human infant's adrenocortical and behavioral responses to stressors and their interrelationships during the neonatal period and later during infancy; (2) examine individual differences in reactivity of the adrenocortical system with regard to social-emotional development; and (3) examine the dynamics of plasma (total) saliva (free, unbound) cortisol and their relations to behavioral responses to stressors and determine the effects of substituting saliva cortisol measures for plasma cortisol measures in research on children. Male and female infants (birth to 12 mos) will serve as subjects. In the experiments with newborns, ecologically relevant stressors such as circumcision, Guthrie blood-tests, weighing and measuring, and discharge examinations will be studied. behavioral state will be examined using a 30-sec coding interval and the Brazelton Neonatal Behavioral Assessment Scale will be used to measure the organization of the newborn's behavior. Activity of the pituitary-adrenocortical system will be examined using measures of plasma and saliva cortisol. Plasma samples will be obtained via heel-stick. In older infants, behavioral and adrenocortical responses to psychosocial stressors such as strangers and separation will be examined. In addition, factors mediating the infant's biobehavioral response to stressors will be explored, including characteristics of mother-infant interaction, maternal perceptions of infant temperament, and maternal support systems. Finally, behavioral correlates of plasma and saliva cortisol indices of adrenocortical activity will be examined in a population of children with phenylketonuria (PKU) during regularly scheduled clinic visits. The proposed experiments will provide basic data in the area of psychobiology as such information pertains to child development and pediatric behavioral medicine. The long-term goals are to gain a better understanding of stress and coping in human infants.